Capacitive pressure sensor

ABSTRACT

Aspects of the disclosure provide a capacitive pressure sensor. The sensor can include a first substrate having a first surface and a second surface, a movable plate at a bottom of a first cavity recessed into the substrate from the first surface, and a second substrate bonded to the first substrate over the first surface. The second substrate includes a fixed plate disposed over the movable plate to form a capacitor. A second cavity is formed between the movable plate and the second surface.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of U.S. ProvisionalApplication No. 62/298,235, “Unique Design and Fabrication Sequence ofMaking Low Cost Capacitive Pressure Sensor with Higher Performance”,filed on Feb. 22, 2016, which is incorporated herein by reference in itsentirety.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Capacitive pressure sensors can be used for measuring low and ultra-lowpressure in a range of a few pascals in many applications. Capacitivepressure sensors are commonly used in, but not limited to, dry airpressure measurement such as sound pressure measurement. There is a needfor further optimizing functionality and performance of capacitivepressure sensors while ensuring a competitive manufacturing cost.

SUMMARY

Aspects of the disclosure provide a capacitive pressure sensor. Thesensor can include a first substrate having a first surface and a secondsurface, a movable plate at a bottom of a first cavity recessed into thesubstrate from the first surface, and a second substrate bonded to thefirst substrate over the first surface. The second substrate includes afixed plate disposed over the movable plate to form a capacitor. Asecond cavity is formed between the movable plate and the secondsurface.

In one example, the fixed plate has a contoured surface facing themovable plate. The contoured surface follows a deflection contour of themovable plate. In another example, the movable plate is contoured andconcaves in a direction away from the fixed plate. In a further example,the movable plate includes a spring structure near an edge of themovable plate.

In one example, the capacitive pressure sensor further includes anisolation layer at a bonding interface between the first substrate andthe second substrate. In another example, the capacitive pressure sensorfurther includes a ventilation path going through the first substrateand the second substrate. The ventilation path includes a first openingat a sidewall of the second cavity and a second opening at a surface ofthe second substrate opposite to a bonding interface between the firstand second substrates.

In one example, the capacitive pressure sensor further includes apackage substrate including an opening, and a cap attached to thepackage substrate enclosing the first substrate and the secondsubstrate. The first substrate is attached to the package substrate atthe second surface of the first substrate with the second cavity overthe opening.

Aspects of the disclosure provide a process for fabricating a capacitivepressure sensor. The process can include forming a first cavity recessedinto a first substrate from a first surface of the first substrate,forming a first isolation layer over the first surface of the firstsubstrate and a surface of the first cavity, forming a diaphragm layerat the bottom of the first cavity over the first isolation layer, andbonding a second substrate to the first substrate over the first surfaceof the first substrate.

In one example, the process can further include reducing the secondsubstrate, forming damping holes in the second substrate over the firstcavity, forming a second cavity in the first substrate between the firstisolation layer and a second surface of the first substrate, andremoving a portion of the first isolation layer between the diaphragmlayer and the second cavity. The process can further includes forming asecond isolation layer over a bonding surface of the second substratebefore bonding the second substrate with the first substrate, andremoving a portion of the second isolation layer above the first cavityafter forming the second cavity.

In one example, the process includes attaching the first substrate to apackage substrate at the second surface of the first substrate, whereinthe package substrate has an opening disposed below the second cavity,and attaching a cap to the package substrate enclosing the firstsubstrate and the second substrate

In one example, fusion bonding is used to bond the second substrate tothe first substrate. In one example, the diaphragm layer is constructedwith silicon oxide or silicon carbide. In one example, the firstsubstrate and/or the second substrate can be made from a prime wafer ora test wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of this disclosure that are proposed as exampleswill be described in detail with reference to the following figures,wherein like numerals reference like elements, and wherein:

FIG. 1 shows an example capacitive pressure sensor according to anexample of the disclosure;

FIG. 2 shows another example capacitive pressure sensor according to anexample of the disclosure;

FIGS. 3A-3B show a bottom-mount package and a top-mount package,respectively, according to some examples;

FIG. 4 shows a conventional capacitive pressure sensor;

FIG. 5 shows a typical surface micromachining process for fabricating aconventional capacitive pressure sensor according to an example;

FIG. 6 shows a fabricating process where a diaphragm sticks to a fixedplate according to an example;

FIG. 7 shows a conventional capacitive pressure sensor according to anexample;

FIG. 8 shows an optimized capacitive pressure sensor according to anexample of the disclosure;

FIG. 9 shows a capacitive pressure sensor according to an example of thedisclosure;

FIG. 10 shows another capacitive pressure sensor according to an exampleof the disclosure;

FIG. 11 shows a diaphragm in a capacitive pressure sensor according toan example;

FIG. 12 shows a capacitive pressure sensor according to an example;

FIG. 13 shows a fabricating process for forming a spring structureaccording to an example;

FIG. 14 shows a capacitive pressure sensor including a thick isolationlayer according to an example;

FIG. 15 shows a capacitive pressure sensor package according to anexample;

FIGS. 16A-16B show a fabricating process according to an example;

FIGS. 17A-17B show another fabricating process according to an example;and

FIG. 18 shows a contact structure for providing electricalinterconnection to a diaphragm according to an example.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an example capacitive pressure sensor 100 according to anexample of the disclosure. The capacitive pressure sensor 100 includes afirst substrate 110, a second substrate 120. The first substrate 110includes a first surface 111 and a second surface 112 opposite to thefirst surface. A thin film diaphragm 130, also referred to as a movableplate, is at a bottom of a cavity 152 recessed into the first substrate110 from the first surface 111 of the first substrate 110. A chamber 151is formed between the diaphragm 130 and the second surface 112 of thefirst substrate 110. The second substrate 120 is bonded to the firstsubstrate 110. The second substrate 120 includes a fixed plate 140disposed over the diaphragm 130 to form a capacitor. A gap 152 (thecavity 152) is formed between the fixed plate 140 and the diaphragm 130.The capacitive pressure sensor 100 further includes an isolation layer160. The isolation layer 160 insulates the first substrate 110 from thesecond substrate 120 preventing electricity passing between the firstand second substrate 110 and 120.

In one example, the first or second substrate 110 or 120 is made from awafer including semiconductor materials, such as silicon, germanium,gallium arsenide (GaAs), and the like. In one example, the diaphragm 130is formed by a thin polysilicon film doped with, for example, Phosphorusions to make the diaphragm 130 conductive. In one example, thickness ofthe diaphragm 130 is in a range of 0.1-10 microns. Various suitablebonding techniques can be used for bonding the first and secondsubstrates 110 and 120. In one example, fusion bonding technique isused. For example, a silicon dioxide layer can be used as an adhesivelayer for the bonding operation. In another example, adhesive materialsare used to bond the two substrates 110 and 120. Various suitableinsulating materials can be used for insulating the first substrate 110from the second substrate 120. In one example, the isolation layer 160is formed by deposition of silicon dioxide.

In one example, the fixed plate 140 is perforated and includes aplurality of damping holes 141. In one example, the capacitive pressorsensor 100 includes a via hole 170 for interconnecting a portion 131 ofthe diaphragm 130 to an electrode (not shown) formed on the capacitivepressor sensor 100. The portion 131 of the diaphragm 131 is connected toother part of the diaphragm 130 through a channel structure 132.

In operation, a sound wave from a sound pressure port propagates throughthe chamber 151 reaching a sensing surface 133 of the diaphragm 130. Thediaphragm 130 vibrates in response to a pressure exerted by the soundwave. The vibration leads to capacitance variation of the capacitorformed by the diaphragm 130 and the fixed plate 140. A current signalcan thus be obtained from a circuit including the capacitor and issupplied as an output of the capacitive pressure sensor 100.

FIG. 2 shows another example capacitive pressure sensor 200 according toan example of the disclosure. The capacitive pressure sensor 200 has astructure similar to that of the capacitive pressure sensor 100.However, the capacitive pressure sensor 100 is configured to be abottom-mount sensor, while the capacitive pressure sensor 200 isconfigured to be a top-mount sensor, with respect to a package structureenclosing the respective capacitive pressure sensor. Bottom-mountpressure sensors or top-mount pressure sensors can be applicable fordifferent applications. As an example, FIGS. 3A and 3B show abottom-mount package 300A and a top-mount package 300B. Both packages300A/300B include a package substrate 320 a/320 b, such as a printcircuit board (PCB), and a cap 310 a/310 b. A sound pressure port 340 ais formed in the package substrate 320 a for the bottom-mount package300A, while a sound pressure port 340 b is formed in the cap 310 b forthe bottom-mount package 300B. The package 300A encloses a bottom-mountcapacitive pressure sensor 330 a having a diaphragm 331 a facingdownward, while the package 300B encloses a top-mount capacitivepressure sensor 330 b having a diaphragm 331 b facing upward.

In FIG. 2, the capacitive pressure sensor 200 includes a first substrate210 and a second substrate 220. A diaphragm 230 (also referred to as amovable plate) is at a bottom of a cavity 252 recessed into the firstsubstrate 210. The first substrate 210 includes a first surface 211 anda second surface 212. A chamber 251 (or cavity) is formed between thediaphragm 230 and the second face 212. The second substrate 220 includesa third surface 221 and a fourth surface 222. The second substrate 220is bonded to the first substrate under the first surface 211 at thethird surface 221. The second substrate 220 includes the fixed plate 240disposed under the diaphragm 230 to form a capacitor. A gap 252 (thecavity 252) is formed between the fixed plate 240 and the diaphragm 230.An isolation layer 260 is formed between the first and second substrate210 and 220 insulating the first substrate 210 from the second substrate220. Similarly, suitable bonding techniques and insulating materials canbe used for bonding or insulating the two substrates 210 and 220,respectively, in various examples.

In addition, a cavity 253 is formed between the fixed plate 240 andfourth surface 222. In one example, the second substrate 220 is madefrom a silicon on insulator wafer. Accordingly, in one example, thesecond substrate 220 includes a first silicon layer 223 made of primequality silicon, an isolation layer 224 made of dioxide silicon, and asecond silicon layer 225 made of bulk silicon. The fixed plate 240 isformed in the first silicon layer 224, while the cavity 253 is formed inthe isolation layer 224 and the second silicon layer 225.

Similarly, in one example, the fixed plate 240 is perforated andincludes a plurality of damping holes 241. In one example, thecapacitive pressor sensor 200 includes a via hole 270 used fromconnecting a portion 231 of the diaphragm 230 to an electrode 271 formedon the second face of the capacitive pressor sensor 200. The portion 231of the diaphragm 230 is connected to other part of the diaphragm 230through a channel structure 232.

The capacitive pressure sensor 200 operates in a way similar to thecapacitive pressure sensor 200. Description of operation of thecapacitive pressure sensor 200 is omitted for simplicity.

The design and fabrication technology of the examples in FIGS. 1 and 2are compatible to any complementary metal oxide semiconductor (CMOS)process. Thus, the capacitive pressure sensors 100 or 200 can bemanufactured in large volume leading to competitive product cost. Inaddition, the design and fabrication technology of the examples in FIGS.1 and 2 can be applicable for various sizes of sensor dies withoutaltering any of design and fabrication concepts.

According to an aspect of the disclosure, design of the capacitivepressure sensors 100 and 200 solves residual stress problems of aconventional capacitive pressure sensor design shown in FIG. 4. FIG. 4shows a conventional capacitive pressure sensor 400. The capacitivepressure sensor 400 includes a first substrate 410, and a secondsubstrate 420. A fixed plate 450 is formed at the first substrate 410. Adiaphragm (movable plate) 440 is formed at a surface of the secondsubstrate 420. For example, the diaphragm 440 can be formed within adevice layer of a silicon on insulator (SOI) wafer. The first substrate410 and the second substrate 420 are bonded together as shown in FIG. 4.The diaphragm 410 is sandwiched between the first and second substrates410 and 420. Due to the bonding process while fabricating the capacitivepressure sensor 400, residual stresses can arise within the device layerthat forms the diaphragm 440. The residual stresses can cause mechanicaldeformation of the diaphragm 440, such as warping or buckling, forexample. The mechanical deformation can significantly affect performanceof the capacitive pressure sensor 400, for example, reducing sensitivityof the capacitive pressure sensor 400.

In contrast, in examples of FIGS. 1 and 2, the diaphragm 130 or 230 aresitting inside a recessed cavity 152 or 252, respectively, andindependent from bonding interface between two substrates, thus avoidingany stresses caused by operations of bonding two substrates.

In addition, due to the recessed-diaphragm structure in the examples ofFIGS. 1 and 2, a complex bonding process for fabricating theconventional capacitive pressure sensor 400 can be avoided. In FIG. 4,the diaphragm 440 is typically made of polysilicon. However, the surfaceof a polysilicon layer is rough due to a deposition process, and thus afine polishing process, for example, a chemical mechanical polishingprocess, is required before fusion bonding of the second substrate 420to the first substrate 410. Additionally, it is difficult to fusion bonda polysilicon surface directly to another silicon or silicon oxidesurface, and further processing processes are needed. In contrast, thediaphragm 130 or 230 is sitting inside a recessed cavity and independentfrom the bonding interface, and no poly silicon layer is formed at anybonding interface. Thus, the complex bonding processes can be avoided.

According to another aspect of the disclosure, the design of thecapacitive pressure sensors 100 and 200 avoids stiction problems ofanother conventional capacitive pressure design. FIG. 5 shows a typicalsurface micromachining process 500 for fabricating a conventionalcapacitive pressure sensor. The surface micromachining process 500includes a sequence of steps S510-S560. At S510, a substrate isprovided. At S520, fixed (back) plate holes are formed. At S530, asacrificial layer 531, for example, of silicon dioxide, is grown ordeposited on the upper surface of the substrate. At S540, a diaphragmlayer is grown over the sacrificial layer 531. At S550, the sacrificiallayer 531 is removed to form a cavity 552 between a diaphragm 551 and afixed plate 553. At S560, a back chamber is etched out below the fixedplate.

During S550, a wet etching process is performed to remove thesacrificial layer 531 to form the cavity 552, which may cause stictionproblems. For example, after the wet etching process, deionized watercan be used to rinse the device being fabricated to remove reminiscentchemicals. Since the gap between the fixed plate 553 and the diaphragm551 is typically less than a few microns, for example, 4 microns, andthickness of the diaphragm 551 is also typically less than a fewmicrons, for example, 2 microns, surface tension of water molecules maycause the diaphragm 551 to stick to the fixed plate 553 as shown in FIG.6. In order to release the diaphragm from stiction, additional processeshave to be performed, such as coating structure surface with surfaceanti-stiction mono-layer (SAM), using super critical carbon dioxide orvapor methanol to release the diaphragm, and the like. The fabricatingprocess 500 is thus complicated.

Compared with the conventional capacitive pressure sensor designemploying the fabricating process 500 in FIG. 5, the design ofcapacitive pressures sensors in FIGS. 1 and 2 simplifies the fabricatingprocess. Specifically, in the examples of FIGS. 1 and 2, the diaphragm130/230 and the cavity (gap) 152/252 can be formed prior to forming thefixed plate 140/240, thus avoiding any wet etching process for formingthe diaphragm 551 in FIG. 5 as well as the stiction problems.

In addition, in the surface micromachining process 500, materials forforming the diaphragm 551 are limited to specific materials in order tosatisfy requirements of the wet etching process employed. For example,the diaphragm layer in the process 500 should not react with wet etchingchemicals used for remove the sacrificial layer 531. Therefore, onlyselected materials can be used for the diaphragm layer. In contrast, thedesigns of FIGS. 1 and 2 examples do not have the above diaphragmmaterial limitation, and any thin film compatible with the design andrespective fabricating process can be employed, such as polysilicon,silicon carbide, and the like.

FIG. 7 shows a conventional capacitive pressure sensor 700 according toan example. As shown, a fixed plate 710 and a diaphragm 720 arepositioned in parallel, and gaps 740 between the fixed plate 710 and thediaphragm 720 are equal at different locations when no pressure is addedto the diaphragm 720. The diaphragm 700 deflects in response to soundpressure 750 exerted on the diaphragm 700 forming a deflection contour730. The deflection contour 730 is a surface convexing towards the fixedplate 710 and indicating a farthest position the diaphragm 720 canreach.

FIG. 8 shows an optimized capacitive pressure sensor 800 according to anexample of the disclosure. The capacitive pressure sensor 800 includes adiaphragm 820 that is flat when stationary and a contoured fixed plate830 forming a capacitor for detecting pressure. The contoured fixedplate 830 includes a contoured surface 831 facing the diaphragm 820 butconcaving in a direction away from the diaphragm 820. The contouredsurface 831 follows a deflection contour 821 of the diaphragm 820. As aresult, an optimized contoured cavity 810 is formed between a fixedplate 830 and a diaphragm 820 where a gap 841 near the edge of thediaphragm 820 is smaller than a gap 842 near the central region of thediaphragm 820.

The above feature of forming a contoured cavity between a fixed plateand a diaphragm in FIG. 8 has several advantages compared with theconventional parallel pate design shown in FIG. 7. First, the featurecan increase sensitivity of a capacitive pressure sensor. For example,in capacitor of FIG. 8 example, regions 851 close to a periphery of thecapacitor have smaller gaps compared with the central area of thecapacitor having the gap 842. Accordingly, compared with a conventionalcapacitive pressure sensor including a plate fixed plate and having agap equal to the gap 842, a same deformation of the diaphragm 820 wouldincur a higher variation of capacitance in the capacitive pressuresensor 800 than in the conventional capacitor. Thus, sensitivity ofrespective capacitive pressure sensor can be improved.

Second, the feature can increase capacitance of the capacitive pressuresensor 800 compared with a conventional pressure sensor having a pair ofparallel plates. As is known, capacitance of a parallel capacitor isproportional to a distance between two plates of the capacitor. Acapacitor with a contoured plate would have a larger capacitance that acapacitor having the same size but including a pair of paralleled platesassuming a maximum gap of the former equals a gap of the latter.Accordingly, a capacitive pressure sensor can maintain its capacitancewhile having a smaller size by implementing the above feature. Reduceddie size leads to decreased cost for a capacitive pressure sensor.

Third, the feature can be used to decrease acoustic noise caused byleakage current through air gap of a capacitive pressure sensor. Forexample, air molecules collisions can create thermionic noise.Increasing distance between two plates in a capacitive pressure sensorcan reduce occurrence of air molecules collisions thus decreasing theleakage current through the air gap. By implementing the contouredcavity technique, in FIG. 8 example, distance between plates at centralpart can be increased while maintaining a desired capacitance value,which helps to reduce acoustic noise.

FIG. 9 shows a capacitive pressure sensor 900 according to an example ofthe disclosure. The capacitive pressure sensor 900 has a structuresimilar to that of the capacitive pressure sensor 100 in FIG. 1 example.For example, the capacitive pressure sensor 900 includes a firstsubstrate 910, a second substrate 920, an isolation layer 960, a fixedplate 940, and a diaphragm 930 that are similar to their counterparts inFIG. 1. The diaphragm 930 is recessed into the first substrate 910.However, different from FIG. 1 example, the capacitive pressure senor900 implements the feature of contoured cavity described herein.Specifically, the fixed plate 940 is contoured and includes a contouredsurface 941 facing the diaphragm 930. The contoured surface 941 concavesinto the fixed plate 940 and follows a deflection contour 931 of thediaphragm 930. In this way, a contoured cavity 952 is formed between thefixed plate 940 and the diaphragm 930.

FIG. 10 shows another capacitive pressure sensor 1000 according to anexample of the disclosure. The capacitive pressure sensor 1000 has astructure similar to that of the capacitive pressure sensor 100 in FIG.1 example. For example, the capacitive pressure sensor 1000 includes afirst substrate 1010, a second substrate 1020, an isolation layer 1060,a fixed plate 1040, and a diaphragm 1030 that are similar to theircounterparts in FIG. 1. The diaphragm 1030 is recessed into the firstsubstrate 1010. However, different from FIG. 1 example, the capacitivepressure senor 1000 implements the feature of contoured cavity describedherein.

Different form the FIG. 9 example, in the capacitive pressure sensor1000, the diaphragm 1030 is contoured instead of the fixed plate 1040.Specifically, the contoured diaphragm 1030 concaves in a direction awayfrom the fixed plate 1040. In this way, a contoured cavity 1052 isformed between the fixed plate 1040 and the contoured diaphragm 1030.The feature of the contoured diaphragm 1030 in FIG. 10 has an effectsimilar to the feature of contoured fixed diaphragm 940, and leads tosimilar advantages described above.

It is noted that, although the technique of forming a contoured fixedplate or a contoured diaphragm is described with reference tobottom-mount configuration examples in FIGS. 9 and 10, the technique canalso be applied to top-mount capacitive pressure sensors.

FIG. 11 shows a diaphragm 1110 in a capacitive pressure sensor accordingto an example. The diaphragm 1110 includes a spring structure 1120, andis anchored to a diaphragm anchoring element 1130 through the springstructure 1120. The spring structure 1120 can buffer or reduce stressimposed on diaphragm 1110 from the anchoring element 1130. For example,structuring bending 1141 of the anchoring element 1130 may cause pull-inforces 1142. The pull-in forces 1142 at different locations with respectto the diaphragm 1110 may have different directions or magnitudes, whichmay cause mechanical deformation of the diaphragm 1110. As describedabove, the mechanical deformation can significantly affect performanceof a capacitive pressure sensor, for example, reducing sensitivity ofthe capacitive pressure sensor. Due to the introduction of the springstructure 1120, sensitivity of the capacitive pressure sensor using thediaphragm 1110 can be improved. Spring structures for anchoring adiaphragm in a capacitive sensor can have various suitable forms. FIG.11 also shows another different design 1121 of a spring structure foranchoring a diaphragm.

FIG. 12 shows a capacitive pressure sensor 1200 according to an example.The capacitive pressure sensor 1200 has a structure similar to theexamples shown in FIGS. 1 and 2. For example, the capacitive pressuresensor 1200 can include a first substrate 1230 and a second substrate1240 bonded together. The second substrate 1240 includes a fixed block1250, while the first substrate 1230 includes a diaphragm 1210 recessedinto the first substrate 1230. However, different from the FIG. 1 orFIG. 2 example, the diaphragm 1210 includes a spring structure 1220. Thespring structure 1220 is similar to the spring structure 1120 in termsof structure and function.

In various examples, the diaphragm 1210 and spring structure 1220 canhave different configurations and structures. In one example, thediaphragm 1210 has a circular shape, and the spring structure 1220 isformed at a ring-shaped region near the edge of the diaphragm 1210. Inanother example, one or more portions of the edge of the diaphragm 1210is anchored to the first substrate 1230 through one or more respectivespring structures, thus the diaphragm 1210 is floating or suspended.

FIG. 13 shows a fabricating process 1300 for forming a spring structureaccording to an example. The fabricating process 1300 can be employed tomanufacture capacitive pressure sensors having a spring structure, suchas the capacitive pressure sensor 1200 in FIG. 12. The fabricatingprocess 1300 includes a sequence of steps S1310-S1326.

At S1310, a wafer is provided. FIG. 13 shows a portion of the wafer formaking one capacitive pressure sensor. The portion corresponds to afirst substrate in a capacitive sensor. At S1312, a masking layer isdeposited over the wafer. At S1314, the masking layer is patterned andetched. At S1316, a recessed cavity is formed by etching into thesubstrate. At S1318, a layer of silicon oxide is grown over thesubstrate. At S1320, the layer of silicon oxide is patterned and etchedto form grooves 1321. At S1322, a polysilicon layer is deposited overthe layer of silicon oxide to form a diaphragm. At S1324, a back chamberis formed below the layer of silicon oxide. At S1326, the layer ofsilicon oxide is etched away, and a spring structure 1326 is formed.

FIG. 14 shows a capacitive pressure sensor 1400 including a thickisolation layer 1440 according to an example. The capacitive pressuresensor 1400 has structures and functions similar to that of thecapacitive pressure sensor 100 in FIG. 1 example. For example, thecapacitive pressure sensor 1400 includes a first substrate 1410 and asecond substrate 1420 bonded together. The second substrate 1420includes a fixed plate 1450, while the first substrate 1410 includes adiaphragm 1430 recessed into the first substrate 1410.

In particular, the capacitive pressure sensor 1400 includes an isolationlayer 1440 at the bonding interface between the first and secondsubstrates 1410/1420. The isolation layer 1440 can insulate the firstsubstrate 1410 from the second substrate 1420, such that leakage currentbetween the first and second substrates 1410/1420 can be reduced oreliminated. In one example, the isolation layer 1440 is formed with athickness that can significantly reduce the leakage current between thefirst and second substrate 1410/1420. In one example, the thickness ofthe isolation layer satisfies the requirement of capacitance of thecapacitor formed by the diaphragm 1430 and the fixed plate 1450. Forexample, the capacitance is determined by a distance 1471 between thediaphragm 1430 and the fixed plate 1450 which is equal to the thicknessof the isolation layer plus a depth 1472 of a cavity 1470 below thefixed plate. The depth 1472 is between a surface 1473 of the firstsubstrate 140 and the upper surface of the diaphragm 1430. Accordingly,a thickness of the isolation layer 1440 can be determined based on thecapacitance wherein capacitance is equal to the area of the platedivided by the distance. In one example, the depth 1472 is designed tobe small with respect to the distance 1471 such that the thickness ofthe isolation layer 1440 can be increased as a higher thickness of theisolation layer 1440 resulting in better noise performance. In anexample, the isolation layer 1440 includes multiple layers of insulatingmaterials, such as a first layer 1441 and a second layer 1442 ofinsulating materials. For example, the first layer 1441 can be a siliconoxide adhesive layer, and the second layer 1441 can be a silicon oxidelayer deposited before forming the diaphragm 1430. In one example, theisolation layer 1440 includes a layer of silicon dioxide.

FIG. 14 also shows a current leakage path 1460. The leakage path 1460starts from a portion 1431 of the diaphragm 1430, and passes through theisolation layer 1441 into the first substrate 1410. In one example, thefirst substrate 1410 is formed by a material having a high impedance.Then, leakage path 1460 passes through the isolation layer 1440 into thesecond substrate 1420. As shown, due to the isolation effect of theisolation layers 1440 and 1441 as well as the high impedance of thefirst substrate 1410, the leakage current between the diaphragm 1430 andthe fixed plate 1450 can be significantly reduced.

A leakage current between a diaphragm and a fixed plate in a capacitivepressure sensor causes acoustic noise to a sensor signal generated fromthe capacitive pressure sensor, and can decrease signal to noise ratio(SNR) of the capacitive pressure sensor. Due to the specific structure(the diaphragm 1430 is recessed into the first substrate 1410) and theaccordingly formed isolation layer 1440, the leakage current of thecapacitive pressure sensor 1400 can be significantly reduced, leading toa high SNR of the capacitive pressure sensor 1400. In one example, acapacitive pressure sensor similar to the example in FIG. 14 can have aSNR of above −70 dB.

In addition, as a larger capacitor can be employed to produce a largersensor signal to increase a SNR, a larger die size is typically requiredin order to obtain a higher SNR. Due to the above isolation structurethat leads to a lower acoustic noise level and a higher SNR, a same SNRcan be obtained without enlarging the die size. Cost incurred by alarger die size can thus be avoided.

FIG. 15 shows a capacitive pressure sensor package 1500 according to anexample. The package 1500 includes a package substrate 1531 and a cap1532 mounted over the package 1531 enclosing a capacitive pressuresensor 1510. The capacitive pressure sensor 1510 has a structure similarto that of the FIG. 1 example. Specifically, the capacitive pressuresensor 1510 has a first substrate 1501 and a second substrate 1502. Thesecond substrate 1502 includes a fixed plate 1511 and the firstsubstrate 1501 includes a recessed diaphragm 1512. The capacitivepressure sensor 1510 is of a bottom-mount sensor, and is mounted to thepackage substrate 1531 with adhesive materials 1515. A back chamber 1513is formed below the diaphragm 1512 and connected with a pressure port1516 created on the package substrate 1531. A chamber 1514 is formedbetween the cap 1532 and the capacitive pressure sensor 1510.

Particularly, a ventilation path 1520 is formed connecting the chamber1514 with the back chamber 1513. Consequently, a reference pressureequal to the atmospheric pressure is provided to the chamber 1514, suchthat either side of the diaphragm 1512 is exposed to a same airpressure, which enables the capacitive pressure sensor 1510 to operatein a gauge measurement mode.

In one example, the ventilation path 1520 includes a first opening 1523at a surface of the second substrate 1502 facing the chamber 1514 and asecond opening 1524 at a side wall of the back chamber 1513. Theventilation path 1520 goes through the first substrate 1501 and thesecond substrate 1502 subsequently as shown in FIG. 15. In addition, theventilation path 1520 is configured to have a certain length such that aphase shift can be created between sound waves reaching different sidesof the diaphragm 1512 from a same source. In this way, the capacitivepressure sensor 1510 can operate properly for dynamic pressureapplications, such as a microphone.

FIGS. 16A-16B show a fabricating process 1600 according to an example.The fabricating process 1600 can be employed to fabricate a bottom-mountcapacitive pressure sensor, such as the example capacitive pressuresensor 100 in FIG. 1. The fabricating process 1600 can include asequence of steps S1610-S1624.

At S1610, a cavity 1631 and a first via hole 1632 is etched into a firstsubstrate 1633. Specifically, the cavity 1631 is recessed into the firstsubstrate 1633 from a first surface 1630 of the first substrate 1633. AtS1612, a first isolation layer 1634, for example, made of silicondioxide, is grown on surface of the first substrate 1633 including thesurface of the cavity 1631. At S1614, a diaphragm layer 1635, forexample, made of polysilicon, is deposited is on the bottoms of thecavity 1631 and the first via hole 1632. It is noted that materials forforming the diaphragm layer 1631 is not limited to polysilicon. Inalternative examples, materials other than polysilicon can be used forthe layer 1631, such as silicon carbide, and the like. The two portionsof the diaphragm layer 1631 at the bottoms of the cavity 1631 and thefirst via hole 1632 are connected through a deposited polysilicon layerat a channel 1636

At S1618, a second substrate 1651 is bonded to the first substrate 1633over the first surface 1630 of the first substrate 1633. The bondingoperation can be fusion bonding or bonding using adhesive materials. Inone example, fusion bonding is used. In one example, a second isolationlayer 1652 of silicon oxide is formed before the boding operation. Inorder to achieve enhanced isolation effect between the first and secondsubstrates 1651 and 1633, the thickness of the second isolation layer1652 can be increased to obtain better isolation.

At S1620, the second substrate 1651 is ground down to desired thickness.At S1622, damping holes 1656 are etched above the cavity 1631, and asecond via hole 1653 is etched above the first via hole 1632. Inaddition, a back chamber 1637 is etched below the cavity 1631. At S1624,part of the first and second isolation layers are removed. Specifically,portions of the second isolation layer 1652 above the cavity 1631 andthe first via hole 1632 are removed, and a portion of the firstisolation layer 1624 below the cavity 1631 is removed. Accordingly, amovable diaphragm 1638 and a fixed plate 1654 are formed, which togetherform a capacitor for pressure measurement. In addition, metals 1655 aredeposited to respective locations to form contacts for electricalinterconnections.

The first and second substrates 1633 and 1651 can be formed with primegrade (device grade) wafer or test grade wafer, both of which arecheaper than SOI wafer. As a result, a capacitive pressure sensor havinga design and structure of FIG. 1 example and fabricated with thefabricating process 1600 can have a lower cost compared with aconventional capacitive pressure sensor similar to FIG. 4 example whereSOI wafers are employed.

It is noted that the above described process 1600 can include otheradditional fabricating steps not shown in FIGS. 16A-16B. For example,the process 1600 can include steps for creating concaved diaphragm orconcaved fixed block to form a concaved cavity between the diaphragm andthe fixed block in order to obtain advantages of respective designs asdescribed above. In addition, the steps of the process 1600 may beperformed in an order different from the example shown in FIGS. 16A-16Bin other examples to realize the same results.

FIGS. 17A-17B show a fabricating process 1700 according to an example.The fabricating process 1700 can be employed to fabricate a top-mountcapacitive pressure sensor, such as the example capacitive pressuresensor 200 in FIG. 2. The fabricating process 1700 can include asequence of steps S1710-S1728.

At S1710, a cavity 1741 and an opening 1742 is etched into a firstsubstrate 1743. Specifically, the cavity 1741 is recessed into the firstsubstrate 1743 from a first surface 1740 of the first substrate 1743. AtS1712, a first isolation layer 1744, for example, made of silicondioxide, is grown on surface of the first substrate 1743 including thesurface of the cavity 1741. At S1714, a diaphragm layer 1745, forexample, made of polysilicon, is deposited is on the bottoms of thecavity 1741 and the opening 1742. It is noted that materials for formingthe layer 1745 is not limited to polysilicon. In alternative examples,materials other than polysilicon can be used for the diaphragm layer1745, such as silicon carbide, and the like. The two portions of thediaphragm layer 1745 at the bottoms of the cavity 1741 and the opening1742 are connected through a channel 1746.

At S1716, a cavity 1751 is formed, for example, by etching, on a secondsubstrate 1753. In one example, the second substrate 1753 is a portionof a silicon on insulator (SOI) wafer, and accordingly includes a secondisolation layer 1752. In one example, the second isolation layer 1752 isformed by a layer of silicon dioxide. As a result of S1716, a fixedplate layer 1754 is formed between bottom of the cavity 1741 and thesecond isolation layer 1752. At 51718, damping holes 1755 are formed inthe fixed plate layer 1754.

At S1720, the first substrate 1743 is bonded to the second substrate1753 with the cavity 1741 adjacent to the cavity 1751. A gap includingthe cavities 1741 and 1751 is formed between the fixed plate layer 1754and the layer 1745. As shown, the first isolation layer 1744 is locatedbetween the first and second substrates 1743 and 1753 insulating thefirst substrate 1743 from the substrate 1753.

At S1722, the first substrate 1743 is reduced from a second surface1746. At S1724, a first via hole 1747 for electrical interconnection tothe fixed plate layer 1754, and a second via hole 1761 for electricalinterconnection to the diaphragm layer 1745 are formed. In addition, acavity 1749 besides the first isolation layer 1744 opposite to the firstcavity 1741 is formed. Further, openings 1748 for forming an isolationwall surrounding the via hole 1761 are formed.

At S1726, a back chamber 1756 (a cavity) is formed below the cavity 1751in the second substrate 1753. At S1728, part of the first and secondisolation layers 1744 and 1752 is removed. Specifically, a portion ofthe first isolation layer 1744 at the bottom of the cavity 1749 isremoved to form a movable diaphragm 1762, while a portion of the secondisolation layer 1752 below the fixed plate layer 1754 is removed to forma fixed plate 1763. The diaphragm 1762 and the fixed plate 1764 form acapacitor for pressure measurement. In addition, portions of the firstisolation layer 1744 at bottom of the via holes 1764 and 1765 areremoved. Further, metallization is performed to form bonding pad 1765and contact 1764 corresponding to the diaphragm 1762 and the fixed plate1763, respectively. The bonding pad 1765 is connected to the diaphragmlayer 1745 through the via hole 1748.

The first substrate 1743 can be formed with prime grade (device grade)wafer or test grade wafer. In addition, the above described process 1700can include other additional fabricating steps not shown in FIGS.17A-17B. For example, the process 1700 can include steps for creatingconcaved diaphragm or concaved fixed block to form a concaved cavitybetween the diaphragm and the fixed block in order to obtain advantagesof respective designs as described above. In addition, the steps of theprocess 1700 may be performed in an order different from the exampleshown in FIGS. 17A-17B in other examples to realize the same results.

FIG. 18 shows a contact structure 1800 for providing electricalinterconnection to a diaphragm according to an example. The contactstructure 1800 is explained with reference to the figure correspondingto S1724 in FIG. 17B (referred to as FIG. S1724 below) which is copiedand shown in FIG. 18. The contact structure 1800 can include one or morevia holes 1820 formed for interconnection to the diaphragm 1745. The viahole 1761 in FIG. S1724 corresponds to one of such via holes 1820. Thecontact structure 1800 can further include an isolation wall 1810surrounding the via holes 1820. The isolation wall 1810 is formed byinsulating materials. The openings 1748 in FIG. S1724 correspond to theleft and right side of the isolation wall 1810. As shown in FIG. S1724,the isolation wall 1810 and the first isolation layer 1744 form anisolation well 1811 insulating contact structures inside the isolationwell 1811 from regions outside the isolation well 1811. The contactstructure 1800 can further includes a bonding pad 1830, for example,formed by an aluminum metallization process. The bonding pad 1830 isconnected with the diaphragm layer 1745 through the via holes 1820. Inone example, a wire bond 1841 can connect to the bonding pad 1830through a ball bonding process.

While aspects of the present disclosure have been described inconjunction with the specific embodiments thereof that are proposed asexamples, alternatives, modifications, and variations to the examplesmay be made. Accordingly, embodiments as set forth herein are intendedto be illustrative and not limiting. There are changes that may be madewithout departing from the scope of the claims set forth below.

What is claimed is:
 1. A capacitive pressure sensor, comprising: a firstsubstrate; a second substrate; a movable plate disposed between a firstcavity recessed into the first substrate and a second cavity recessedinto the first substrate; and a ventilation path including a firstopening at a sidewall of the second cavity and a second opening at asurface of the second substrate opposite to a bonding interface betweenthe first and second substrates, the ventilation path going through thefirst substrate and the second substrate, wherein the second substrateis bonded to the first substrate and includes a fixed plate disposedover the first cavity and the movable plate to form a capacitor.
 2. Thecapacitive pressure sensor of claim 1, wherein the fixed plate has acontoured surface facing the movable plate, the contoured surfacecorresponding to a deflection contour of the movable plate.
 3. Thecapacitive pressure sensor of claim 1, wherein the movable plate iscontoured and concaves in a direction away from the fixed plate.
 4. Thecapacitive pressure sensor of claim 1, wherein the movable plateincludes a spring structure disposed near an edge of the movable plateand configured to reduce stress on the movable plate.
 5. The capacitivepressure sensor of claim 1, further comprising: an isolation layer at abonding interface between the first substrate and the second substrate.6. A capacitive pressure sensor package, comprising: the capacitivepressure sensor of claim 1; a package substrate including an opening,wherein the first substrate is attached to the package substrate withthe second cavity over the opening; and a cap attached to the packagesubstrate to enclose the first substrate and the second substrate.
 7. Aprocess for fabricating a capacitive pressure sensor, comprising:forming a first cavity recessed into a first substrate from a firstsurface of the first substrate; forming a first isolation layer over thefirst surface of the first substrate and a surface of the first cavity;forming a diaphragm layer at the bottom of the first cavity over thefirst isolation layer; and bonding a second substrate to the firstsubstrate over the first surface of the first substrate.
 8. The processof claim 7, further comprising: reducing the second substrate; andforming damping holes in the second substrate over the first cavity. 9.The process of claim 8, further comprising: forming a second cavity inthe first substrate between the first isolation layer and a secondsurface of the first substrate; and removing a portion of the firstisolation layer between the diaphragm layer and the second cavity. 10.The process of claim 9, further comprising: forming a second isolationlayer over a bonding surface of the second substrate before bonding thesecond substrate with the first substrate; and removing a portion of thesecond isolation layer above the first cavity after forming the secondcavity.
 11. The process of claim 9, further including: attaching thefirst substrate to a package substrate at the second surface of thefirst substrate, wherein the package substrate has an opening disposedbelow the second cavity; and attaching a cap to the package substrate toenclose the first substrate and the second substrate.
 12. The process ofclaim 7, wherein bonding the second substrate to the first substrateincludes fusion bonding the second substrate to the first substrate. 13.The process of claim 7, wherein the diaphragm layer is constructed withsilicon oxide or silicon carbide.
 14. The process of claim 7, whereinthe first substrate is a portion of a prime wafer or a test wafer, andthe second substrate is a portion of a prime wafer or a test wafer.